1. Field of the Invention
The present invention relates to a liquid crystal display module, and more particularly to a liquid crystal display module that can be made lightweight with reduced light loss.
2. Description of the Related Art
Generally, a liquid crystal display (hereinafter, referred to as LCD) displays a desired picture on a screen by controlling the transmittance of the light supplied from a backlight unit by using a liquid crystal display panel including a plurality of liquid crystal cells arranged in a matrix and a plurality of control switches to switch video signals to be supplied to each of the liquid crystal cells.
FIG. 1 is a perspective view representing a known liquid crystal display module.
Referring to FIG. 1, the liquid crystal display module includes a lamp 22 to generate light, a lamp housing 4 where the lamp 22 is equipped, a light guide panel 2 to convert an incident light from the lamp 22 into a surface light, optical sheets 8, 10, 12 and 14 stuck to the light guide panel 2 to increase the efficiency of the light incident to a display panel, a reflective plate 6 stuck to the rear surface of the light guide panel 2 to reflect the light emitted through the rear surface of the light guide panel 2, and a liquid crystal display panel 20 to realize a picture in use of the light generated at the lamp 22.
The lamp 22 is mainly a cold cathode fluorescent lamp. And the light generated at the lamp 22 is incident to the light guide panel 2 through the incident surface that exists at the side of the light guide panel 2.
The lamp housing 4 is installed to have a reflective surface in the inner surface thereof in a manner of covering the lamp 22, so as to reflect the light from the lamp 22 to the incidence surface of the liquid guide panel 2.
The light guide panel 2 converts the incident light from the lamp 22 into the surface light. Such a light guide panel 2 has a sloped lower surface and a horizontal upper surface, which are perpendicular to each other. The lower surface of the light guide panel 2 has a reflective plate 6. The light guide panel 2 transmits the incident light from the lamp 22 towards the end opposing the end to which the lamp 22 is attached. The light guide panel 2 is made of polymethyl methacrylate (PMMA), which has a good transmissivity and has high degree of strength, so that it is not easily changed or broken.
The light incident on the reflective plate 6 is reflected through the rear surface of the light guide panel 2 to the light guide panel 2, thereby reducing light loss.
If the light from the lamp 22 is incident to the light guide panel 2, it is reflected at the lower surface, which is sloped at a specific tilt angle so the light progresses evenly toward the outgoing surface. The light, which progresses to the lower surface and the side surface of the light guide panel 2, is reflected to the reflective plate 6 to progress toward the outgoing surface. The light emitted through the outgoing surface of the light guide panel 2 is diffused to the whole area by the diffusion sheet 8. On the other hand, the light incident to the liquid crystal display panel 20 has the highest light efficiency when it is perpendicular. For this, two prism sheets 10 and 12 are stacked in order to make the progress angle of the light emitted from the light guide panel 2, and the light is perpendicular to the liquid crystal display panel 2.
First and second prism sheets 10 and 12 are composed of a plurality of prism arrays with peaks and valleys. The two prism sheets 10, 12 concentrate the outgoing light from the diffusion sheet 8 in a direction perpendicular to the lower planar surface.
A protective film 14 is used to protect the surface of the second prism sheet 12, and diffuse the light to evenly distribute the light.
In this way, the light generated at the backlight unit is incident to the liquid crystal display panel 20.
The liquid crystal display panel 20 is formed between an upper polarizer 18 and a lower polarizer 16. It has liquid crystal cells arranged between upper and lower substrates 20A, 20B in an active matrix. And the liquid crystal display panel 20 has a thin film transistor installed at each of the liquid crystal cells to switch video signals. The refractive index of each liquid crystal cell is changed in accordance with the video signal, so a picture corresponding to the video signal is displayed. That is, the liquid crystal panel 20 displays a picture by selectively transmitting the light from the backlight unit by way of having liquid crystal driven by a voltage difference between a common electrode of the upper substrate 20A and the a pixel electrode of the lower substrate 20B.
The first and second prism sheets 10 and 12 and a diffusion sheet 8 among the optical sheets of the liquid crystal display module include a function layer where the optical sheet is actually functional (i.e. provides the functional features), and a support layer which supports the function layer. For the diffusion sheet 8, a function layer of several tens μm and below which is a dispersion layer is located on a support layer of 100˜220 μm. For the first and second prism sheet 10 and 12, a prism pattern which is a function layer of several tens μm is located on a support layer of up to 100 μm. There is a problem in that the overall thickness of the liquid crystal display module gets thicker due to the supporting layer. In order to solve such a problem, in the known liquid crystal display module, lightness and high brightness have been studied, especially the lightness of backlight unit has been actively studied.
For example, a liquid crystal display module has been suggested in which the light guide panel and the optical sheets are integrated as shown in FIG. 2, and a liquid crystal display module where the light guide panel and the optical sheets are removed as shown in FIG. 3.
The liquid crystal display module shown in FIG. 2 forms optical sheets 30 on the light guide panel 2, thereby reducing the thickness of the backlight unit since it is not necessary to form a support layer for the optical sheets 30. However, there is a problem in that light loss occurs when the outgoing light from the light guide panel 2 repeatedly passes through the optical sheets 30 and the polarizer 16 as shown in FIG. 2. This is because a total reflection condition established by a gap between the light guide panel 2 and the optical sheets 30 can not be established between the light guide panel 2 and the other optical sheets 30 stuck thereto.
The liquid crystal display module shown in FIG. 3 may reduce the thickness of the backlight unit by constituting a light guide panel with the lower substrate of the liquid crystal display panel without any separate light guide panel. However, in case that the light is incident from the lamp 22 that is located at one side of the lower substrate 20B, there is a problem in that light loss occurs when the light generated at the lamp 22 repeatedly passes through the optical sheets 26, the polarizer 16, lower patterns (thin film transistor, signal line and pixel electrode) formed on the polarizer 16 and upper patterns (common electrode, color filter and black matrix) formed on the upper substrate 20A.